Flash tanks are common pieces of equipment utilized in many chemical and industrial processes. The flash tank is typically used to recover steam from a process fluid, to utilize the steam in other portions of the process, and also allow recycle or other disposition of process liquid with which the steam may be mixed. One typical industrial process utilizing one or more flash tanks is the digestion of comminuted cellulosic fibrous material to form paper pulp or the like. Typical utilization of flash tanks for such a process may be seen in U.S. Pat. Nos. 3,362,868 and 3,414,038.
Conventional flash tanks usually have far from ideal operation. Foam is generated within the flash tank, and if the foam generation is too great, the foam can be carried out of the flash tank with the flashed steam. Foam in the flashed steam makes that steam unsuitable for further utilization in the process system (e.g. unsuitable for use in steaming wood chips to be supplied to a continuous pulp digester). It has always been thought that foam was inherently formed when the feed liquor was introduced into the flash tank, and the most common attempt to prevent foam being carried from the flash tank was to provide a spiral arrangement, referred to as a cyclone separator, at the top of the flash tank. However under many circumstances foam generation is still undesirably high, often resulting in substantial down time of the chemical or industrial process (e.g. shut down of the continuous pulp digester).
According to the present invention it has now been recognized that the source of the foaming problem in flash tanks is the splashing of feed fluid in the liquid seal maintained within the flash tank. The maintenance of a liquid seal at the liquid outlet from the flash tank is necessary in order to prevent steam from being carried with liquid from the flash tank to a subsequent processing station (e.g. another flash tank, chemical recovery, or the like). To the extent that the splashing action can be minimized the density of any foam generated can be increased, and the higher the density of the foam the less likelihood there is that any of it will be carried out with the steam through the steam vent. Also, according to the present invention it has been found that other mechanisms can be utilized for enhancing the density of the generated foam, further minimizing the possibility that foam will be carried out the steam vent.
According to one aspect of the present invention a method of flashing steam in a vertically disposed flash tank, having a feed fluid inlet, a steam outlet, and a liquid outlet, is provided. The method comprises the following steps:
(a) Continuously feeding feed fluid under pressure into the inlet. Typical feed fluid might be the black liquor from a continuous pulp digester. PA0 (b) In the flash tank, reducing the pressure acting on the feed fluid so that steam and liquid are generated. PA0 (c) Removing the generated steam through the steam outlet. The steam is subsequently re-utilized in the chemical or industrial process, such as by passing it to a steaming vessel for steaming wood chips ultimately to be fed to the pulp digester. PA0 (d) Removing the liquid through the liquid outlet. The removed liquid may, for example, in a cellulosic pulping process be passed to a second flash tank, and ultimately to a black liquor storage or regeneration station. PA0 (e) Maintaining a liquid seal adjacent the liquid outlet so that no steam passes through the liquid outlet with the liquid. If steam passes with the liquid to the next station it greatly adversely affects the next station, and may require shut down of the entire process. PA0 (f) positively preventing feed fluid from directly impacting liquid in the liquid seal so that foam generated by feed fluid splashing in the liquid is minimized, and so that foam will not pass out of the steam outlet. Foam passing out the steam outlet renders the steam sought to be recovered unusable for its desired functions.
And,
The flash tank according to the present invention is utilizable in practicing the abovedescribed method, and may be constructed in two primary manners. The feed fluid may be introduced downwardly through the top of the vessel to impact upon a conical deflecting member, and from the conical deflecting member be deflected onto the conically downwardly sloping walls of a bottom portion of the flash tank. Another way is to provide a liquid holding tank externally of the flash tank, the liquid seal being provided in the liquid holding tank. The feed fluid is introduced upwardly in the tank, impacts upon a shallow dish deflecting plate, and from the deflecting plate is deflected onto the conically downwardly sloping walls of the flash tank bottom portion. The liquid holding tank preferably extends directly downwardly from the flash tank liquid outlet, and the cross-sectional dimensions thereof are provided so that the normal impacting pattern of the deflected feed fluid is onto the flash tank walls, not in the liquid seal. The external liquid holding tank arrangement can also be provided with the downwardly directed feed fluid conduit with conical deflecting member.
It is the primary object of the present invention to provide a flash tank, and method of flashing steam utilizing such a tank, with minimum possibilities that foam from the tank will contaminate the flashed steam. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.